Method and device for monitoring and improving arteriogenesis

ABSTRACT

A method for determining an arteriovascular condition of a subject having an arterial blood flow is shown. The method involves determining a temporal progression of an instantaneous blood flow condition of the arterial blood flow as well as deriving a slew rate of the temporal progression during an increase of the temporal progression. In addition, an arteriovascular condition indicator device is shown, which comprises: an input for receiving an input signal representing an instantaneous arterial blood flow condition of a subject and a slew rate monitor connected to the input. A corresponding control device for providing an activation signal is also shown. The control device comprises a maximum detector connected to the slew rate monitor. A method for stimulation of arteriogenesis is also shown, wherein a temporal progression of an instantaneous blood flow condition is monitored, a slew rate of the temporal progression is derived, and the maximum of the slew rate is determined. An external pressure is applied repeatedly to the arteriovascular section in synchronization with the occurrence of the determined maximum.

TECHNICAL FIELD

The present invention relates to devices and methods for sensing thearteriogenic capacity of a patient. The arteriogenic capacity reflectsthe increase of diameter and/or elastic properties of arterial vessels.In case of pathological properties of particular artery sections,respective treatments are focused on improving these properties byarteriogenesis.

In order to stimulate arteriogenesis improving an arteriovascularcondition in an arteriovascular section of a patient prior artapproaches are focused on the shear stress of artery vessels generatedby the blood flow in the vessel. Studies pertaining to the prior arthave suggested to increase the shear stress for training the vessels inpathologic vessel sections for training the vessel section. However,these approaches focused on training the particular vessel section byincreasing the shear stress on the vessel have not led to satisfactoryresults. In other words, stimulating the arteriogenesis by training withhigh shear stresses has not resulted in reliable training increases. Inaddition, taking the shear stress as a measure for determining thetraining performance according to the prior art approaches has not ledto a significant and reliable measure for the arteriogenic capacity.

It is therefore an object of the invention, to provide a method and adevice for providing an accurate and reliable measure for arteriogeniccapacity as well as of the actual status of the arteries. Further, it isan object of the invention to provide a method and a device enabling aneffective training of the arteriogenesis of a subject.

SUMMARY OF THE INVENTION

This object is solved by the inventive method for determining anarteriogenic capacity, by the arteriogenic capacity indicator device,the control device as well as the actuator device as defined in theindependent claims.

It is the concept underlying the invention to take the variation of theblood flow condition in the artery section as a means for determiningthe status of the vessel section as well as to take the variation of theblood flow condition actively to increase the arteriogenic capacity byselectively and actively manipulating the variation of the blood flowcondition.

The blood flow condition relates to the arterial blood flow, which isinherently linked with the shear stress exerted on the inner vesselsurface of the arteriovascular section. In contrast to the prior artwhich is focused on the blood flow condition itself, i.e. on the shearstress, the invention is focused on the variation, i.e. the temporalprogression of an (instantaneous) blood flow condition. As blood flowcondition, blood flow velocity, blood flow rate, fluid pressure orshearing force within the arteriovascular section is used as physicalquantity. Due to the properties of the vessel, i.e. the properties ofthe arteriovascular section and the hydrodynamic properties within thearterial vessel, these physical quantities are closely related to eachother. According to a preferred model, blood flow velocity, blood flowrate, fluid pressure and shearing force are proportional to each other.The proportionality is given by parameters like blood viscosity, vesseldimensions and relationships among the blood vessel dimensions and thelike. In the context of the invention, the term vascular section relatesto an arteriovascular section and reflects the inner space delimited bythe vessel section as well as the (mechanic) reaction of the vascularsection, i.e. elastic and plastic properties of the respective tissue(or vessel) delimiting the arteriovascular section. With regard to theblood flow condition, the term vessel is equivalent to the term vascularsection and vice versa. Further, the invention relates to the arterialpart of the vascular system of a subject.

According to the invention, an arteriovascular condition in a pathologicsection of the arteriovascular system of the subject is determined basedon a blood flow condition. A temporal progression a blood flow conditionis determined using instantaneous values representing the arterial bloodflow. In this respect, the arterial blood flow can be represented by theinstantaneous pressure, instantaneous flow velocity, instantaneous flowrate, instantaneous shearing force or by quantities representing thesephysical quantities. The (arterio-)vascular condition is determined bythe slew rate or by another quantity representing the dynamic variationsof the temporal progression. The positive variation of the temporalprogression, i.e. an increase of the temporal progression is used forderiving the slew rate. The term slew rate relates to a variety ofmeasures and represents any variation of the blood flow condition.According to a preferred embodiment, the slew rate is determined byderiving the progression of the blood flow condition with respect totime. This relates to time continuous as well to time discrete temporalprogressions. In a similar embodiment, the slew rate is determined byrelating the increase of the progression to the distance of time spannedby the increase. Thus, the slew rate represents the ascending radiant orslope or increase of a straight line between two measure points of theblood flow condition.

In other embodiments, the slew rate represents the maximum frequency or(the power or energy of) maximum frequencies of a representation of thetemporal progression in frequency-domain. Since the maximum frequenciesrepresent the maximum slew rate (or the highest slew rates) of thetemporal progression, the maximum frequency in the frequency-domain canbe used as a measure for the (maximum) slew rate. In particular, theenergy or power represented by the area of the progression in thefrequency-domain for high frequencies can be used as a measure for theslew rate. The area is enclosed between abscissa and the progression(curve) in frequency-domain for frequencies greater than a minimumfrequency at which the progression has a minimum amplitude, the minimumamplitude relating to the highest values of increase in time-domain. Theamplitude can be a fixed value or can be related on the complete energyof the progression signal, for example 5% of the total energy of theprogression in frequency-domain. In addition, the temporal progression(e.g. as a result of direct or indirect arterial pressure measurements)can be pre-filtered by a low-pass filter or band-pass filter prior tothe determination of a measure of the slew rate. Such filters are usedto block frequencies not relevant to the determination of the slew rate.The band-pass filter is adapted for passing frequencies, which can beexpected for the progression during the systole phase of the arterialblood flow. The low-pass filter is adapted for passing these frequenciesoccurring during the systole phase and for blocking higher frequencies,e.g. frequencies of noise resulting from the blood flow and the vascularsystem, in particular resulting from turbulences. Further, a high-passfilter can be used for blocking noise resulting from external impactsonto a sensor measuring the pressure or onto the vascular system.

Further embodiments relate to the shape of the temporal progression byproviding a set of predefined (increasing) curve shapes and respectiveslew rate values and by matching the determined temporal progressionwith the curve shape. Any suitable matching or best-fit method can beused to determine the curve shape (of the predefined curve shapes),which shows the highest similarity with the determined (measured)progression. In addition, interpolation between slew rate values can beused to determine a slew rate corresponding to the temporal progression,the determination being based on the predefined curve shapes, similaritymeasures between predefined curve shapes and the temporal progression,and the respective slew rate values of the curve shapes. The higher thesimilarity, the higher the impact of the corresponding slew rate valueson the resulting slew rate, which is to be determined.

In general, according to the invention, the temporal progression duringthe systole is relevant to the slew rate, i.e. the invention is based onthe observation and determination of the positive slew rates.

Alternatively, it is also possible to determine the slew rates andespecially the PSI and RPSI (see below) during the diastole. Thisembodiment is especially useful in case that an external pressure isapplied during the diastole.

The determination of the slew rate is based on measurements of a bloodflow condition, the blood flow condition being one of velocity, pressureor shearing force. According to the invention, only one of thesephysical quantities is measured and provides the values of the temporalprogression of the blood flow condition. However, in order to derive theprogression from the physical quantities, a predefined function isgiven, for example a normalizing function or a function deriving one ofthe physical quantities velocity, pressure or shearing force from adistinct one measured physical quantity. For example, the shearing forcecan be derived from the blood flow velocity, the blood flow rate, or thefluid pressure according to the predefined function. The predefinedfunction can represent physical relationships among the physicalquantities, can represent physical parameters of the fluid path providedby the vascular section, can provide fluid mechanical interrelations ora combination thereof. In a particular preferred embodiment, a measurefor providing the blood flow condition is used, which does not depend onthe actual diameter of the respective vascular section, i.e. a bloodflow condition which does not require measurements of the vesseldiameter or other vessel dimensions. Such measures can be provided bydetermining a value depending on geometric vessel properties andassuming or predefining a constant value which reflects the geometricvessel, i.e. properties of the dimensions of the vascular section andthe corresponding fluid dynamic properties, e.g. properties concerningthe distribution of pressure or velocity within the cross section of thevascular section. Dependencies on the diameter can be eliminated fromthe determined value by normalizing a determined value with thepredefined diameter (or other vessel dimensions relevant to the flow),i.e. by dividing the determined value by a value equal to or directproportional to the predefined diameter.

Accordingly, the maximum of the slew rate is one of the maximum slewrate of the blood flow velocity, the blood flow rate, the fluid pressureor the shearing force. Further, the maximum slew rate can be normalized.The normalization basis can be provided by a mean value of the bloodflow velocity, of the blood flow rate, of the fluid pressure or of theshearing force (as an average over time). Further, the normalizationbasis can be provided by a dimension of the blood flow, for example across section of the pertaining vascular section. In particular, thenormalization basis can be provided by the mean value as described abovedivided by the cross section diameter of the pertaining vascularsection. Further, the normalization basis can be related to apredetermined value, for example by multiplication of one of theabove-mentioned normalization basis values with a predetermined value.If the predetermined value is (positive and) smaller than one, thenormalization basis is formed by a fraction of the above-mentionednormalization values.

According to a preferred model, changes in velocity, volume flow rate,shear stress and shear rate are mutually proportional since vasculargeometry and blood viscosity is given (i.e. is constant). In oneembodiment, the maximum slew rate is represented by the peak velocityincrease, PVI, which is the (positive) maximum of the slew rate of theblood flow velocity. Preferably, this peak velocity increase isnormalized by division by the mean flow velocity, which is the flowvelocity averaged over time, preferably averaged over a plurality ofpulses. The resulting value is called relative peak velocity increase,RPVI. PVI as well as RPVI both are directly based on the blood flowvelocity as underlying physical quantity. However, since the shearingforce (in particular the slew rate thereof) is relevant to thearteriovascular condition, a peak shear increase (peak shearing forceincrease) is derived based on the PVI or RPVI. As interrelationshipbetween velocity-based quantities and shearing force-based quantities, aproportionality factor is given, which is a function of the vesseldiameter, the blood viscosity, the radial hematocrit and velocityprofile. In a simplified model, vessel diameter, blood viscosity, radialhematocrit profile and velocity profile are combined to one constantvalue. Due to this proportionality between flow velocity and shear rateor physical stress, values for a relative peak shear increase (RPSI),i.e. the maximum shearing force increase normalized by a mean value ofthe shearing rate, can be provided identical to respective values of therelative peak velocity increase. In this respect, the term peak isequivalent to maximum as defined above, i.e. the positive peak. The termincrease is equivalent to the slew rate as defined above. The relativepeak shear increase (=peak shear increase normalized by its mean value)can be further normalized by division by the mean pseudo shear rate, togenerate a dimensionless parameter. In particular, this parameter givesa measure, which does not depend on the diameter of the vascular section(or other geometrical properties of the inner tubular room delimited bythe vessel. The mean pseudo shear rate is given (according to thissimplified model) as 8× ν/d, wherein ν is the mean blood velocity(averaged over time), and d is the diameter of the pertaining vessel.The resulting value is dimensionless and is denoted pulse shear index,PSI. Due to the normalization by the vessel diameter, additionalmeasurements or assumptions are necessary for providing the mean pseudoshear rate. Further, the normalization characterizes the pulsatilehemodymanic conditions of a vessel as a single number.

Consequently, in a preferred embodiment of the method for determining anarteriovascular condition of the invention, the RPSI or PSI isdetermined.

According to the invention, one of these quantities, in particular theRPSI or the PSI, reflecting the temporal progression of the blood flowcondition can be used (i) for passively determining an arteriovascularcondition and (ii) as a measure for controlling a device activelytraining the arteriovascular system of a subject. For passivelydetermining the arteriovascular condition, a value representing themaximum slew rate of the temporal progression (or an average thereof) isdetermined once. Based on this number, the arteriovascular capability isof the subject (in particular of the pertaining part of the vascularsystem) can be estimated. In addition, a healing process or a trainingprogress can be determined by repeatedly determining the arteriovascularcondition, for example in time intervals greater than 3 days, 5 days or10 days, or greater than 3 days and less than 30 days, preferablybetween 5 days and 14 days and, in a particular embodiment, about 10days. Between the values representing the maximum of the slew rate (thatis the values representing the measured or derived physical quantities)and the arteriovascular condition (given in form of a grade or anindex), a simple relationship is given. This relationship is representedby a monotone increasing relationship or a function such that, for highslew rates, a good or a high arteriovascular condition is given and forlow maximum slew rates, a poor, i.e. a low arteriovascular condition isgiven. According to the repeated measurement results, an arteriogeniccapacity can be derived, which is also in monotone relationship with theincrease of the arteriovascular condition. That is, if thearteriovascular condition increases significantly in the cause of therepeated measurements, a high arteriogenic capacity of the subject isprovided. However, if the arteriovascular condition only shows poorimprovement, the arteriogenic capacity of the subject is low.Preferably, the arteriogenic capacity is provided by rates or numbers,for example increasing with improving arteriogenic capacities.

The embodiments for determining the arteriovascular condition andactively training the arteriovascular system described above can be usedfor monitoring the physical training of athletes or achievement ofindividuals seeking to improve the physical fitness. In particular, theembodiments relate to non-therapeutic applications and applications forindividuals, which do not suffer from cardiovascular diseases. Ingeneral, according to one aspect of the invention, the actuation deviceaccording to the invention. is used for stimulation of arteriogenesis asdescribed herein.

According to a preferred embodiment, a value for a vascular condition isbased on a plurality of measurements or determinations. In particular,plurality of consecutive individual arteriovascular conditions aredetermined and averaged for providing a resulting (averaged) vascularcondition. The consecutive individual arteriovascular conditions can beprovided by a number of peak measurements, each peak providing one ofthe individual arteriovascular conditions. By averaging, random errorscan be reduced. According to an embodiment, a measurement time intervalis given, for example half an hour, 15 minutes, or 1 minute or less than30 sec., during which the arterial blood flow is continuously monitored,each peak (each heart beat) providing the basis for deriving a slew rateof the respective temporal progression of the measured quantity. In thecourse of the measurement time interval, all individual maximum slewrates are given as a total or sum. Instead of a measurement timeinterval, a predefined number of a heart beats can be given. In thiscase, the maximum slew rates or the respective values reflecting thearteriovascular conditions of the predefined number of heart beats areaveraged.

According to the invention, the measurements are carried out by laserDoppler velocimetry, sonography or magnetic resonance imaging. Othermeasurements are based on pressure sensoring, wrist blood pressuremonitoring, finger blood pressure monitoring, sphygmomanometry,plethysmometry, plethysmography, IR-plethysmography, or intravascularblood pressure sensing. The blood pressure can be measured by externalpressure of volume variation measurements at the skin region, underwhich the vascular section is located. In this regard, microphonesdirectly contacting the skin can be used or other instruments monitoringthe movement of a medium, the medium being in contact with the vascularsystem. Even though only relative movements can be used and absolutevalues of the blood pressure might include a significant error, ingeneral, all measurements suitable for determining the point of time atwhich the maximum slew rate occurs can be used with the invention. Othersuitable measurements are impedance measurements, i.e. measurements ofthe complex electrical impedance of the vascular section, of the tissuesurrounding the vascular section or of the skin (and the underlyingtissue), under which the vascular section is located. Such measurementsare carried out by applying an AC current (or AC voltage) and measuringthe phase and/or frequency distribution of the resulting AC voltage (orcurrent) according to Ohm's law. Further, the myocardial impedance canbe measured, in particular for determining the point of time, at whichthe maximum slew rate occurs. These measurements directly or indirectlymeasure a physical quantity, from which the blood flow condition isderived, or which forms the basis for determining the blood flowcondition based on an other physical quantity. According to a preferredembodiment, laser Doppler velocimetry is used, which includesmeasurements of phase shifts and/or frequency shifts between receivedand emitted light and the transformation of the phase shifts into a flowvelocity. Thus, the quantity directly measured in this embodiment is aphase or an amplitude of light and the resulting physical quantity isflow velocity. In addition, if the blood flow velocity is measured (asin the embodiment described above), the shearing force can be derivedtherefrom (by a proportional relationship or by another monotonefunction).

In a first aspect of the invention, the slew rate is derived as ameasure for the arteriovascular condition as described above. Accordingto a second aspect, the time of occurrence of the maximum slew rate isdetermined for providing the point of time of maximum hydromechanicalburden on the pertaining arteriovascular section of the subject. Thispoint of time is critical to active training of the arteriogeniccapacity. Thus, the temporal progression of the instantaneous blood flowcondition is monitored to determine the point of time of maximum slewrate for activating a supporting mechanism. This supporting mechanismadditionally increases the slew rate by appropriate intervention to thepertaining arteriovascular section. This supporting mechanism isprovided by an actuator device which exerts pressure onto thearteriovascular section which is to be trained. The actuator device cancomprise an apparatus for external pneumatic counterpulsation (ECP) usedin the prior art to support the transfer of blood from extremities tothe heart for reducing symptoms of ischemia. In contrast to thesedevices known from the prior art, the actuator device is activatedduring the systole for additionally increasing the blood pressureresulting in additional maximum slew rates leading to increasedarteriogenesis.

The method of the present invention can be performed based on dataobtained from any body region where arteries are present. This includese.g. the base of the tongue.

The present invention provides an arteriovascular condition indicatordevice for sensing, determining and indicating the point of time ofmaximum slew rate, a control device for providing a respectiveactivation signal upon detection of the point of time of maximum slewrate as well as an activator device, which can be controlled by thecontrol device. While the indicator device is focused on indicating thevalue representing the vascular condition, a similar control device isfocused on determining the point of time of maximum slew rate in orderto appropriately (delayed or undelayed) activate the actuator device.Due to the similar aspects of the control device and the indicatordevice, most of the components are identical. Further, both devices canbe provided as one common apparatus, which selectively provides eitheran indication of the vascular condition or the point of time of maximumslew rate, or provides both.

An indicator device for indicating an arteriovascular conditioncomprises an input for receiving an input signal representing the bloodflow condition. This input signal is preferably a digital or analogoussensor signal received from a sensor from a measuring device. Inaddition, the device comprises a slew rate monitor which is connected tothe input wherein the slew rate monitor processes the input signal andprovides the (actual) slew rate of the blood flow condition representedby the input signal. The slew rate monitor detects the differencebetween two (consecutive) blood flow conditions and provides the ratioof this difference to the corresponding distance of time passed betweenthe two blood flow conditions. Further, the slew rate monitor canprovide a differentiation of the blood flow condition with respect totime by an analog or digital derivation block or circuit. In addition,the slew rate monitor can derive a value representing the slew ratebased on a representation of the blood flow condition in thefrequency-domain. Further, a maximum detector is provided receiving theoutput of the slew rate monitor. The maximum detector receives theinstantaneous slew rates and provides the (positive) maximum of the slewrates as maximum slew rate. Therefore, the maximum detector haspreferably a comparator which allows to compare the slew rates and toidentify the maximum slew rate. Further, the maximum detector can haveanother derivation block, the derivation block receiving the output ofthe slew rate monitor and provides a differentiation of the slew ratewith respect to the time. This corresponds to a second differentiationwith respect to time. At the maximum slew rate, the second derivative(=result of the second differentiation) is crossing zero. Thus, themaximum slew rate can be determined by monitoring zero crossings of thesecond differentiation of the blood flow condition with respect to timewhereby the maximum slew rate is defined by the slew rate occurring atthe zero crossing of the second derivative.

The maximum slew rate is provided to an output element of the indicatordevice, the output element outputting a representation of the maximumslew rate as an indicator for the arteriovascular condition. The maximumslew rate can be provided as a displayed nominal value or can beprovided by a graphical illustration. In addition, the output elementcan provide an analog or digital signal having a value which reflectsthe maximum slew rate. Depending on the focus and desired function ofthe device, the output element can output the maximum slew rate togetherwith the time of occurrence. The indicator device further comprises amemory for buffering the maximum slew rate, which is overwritten eachtime a new slew rate is determined, each time, a higher maximum slewrate is determined or a new average value is provided by an averagingunit of the indicator device, the averaging unit averaging the maximumslew rates.

In another embodiment, a display is adapted or controlled for imagingthe (maximum) slew rate or a distribution of (maximum) slew rates at aposition of the display, which corresponds to the location of thevascular section or sections corresponding to the slew rate or rates.Preferably, the (maximum) slew rate is determined for a plurality ofvascular sections and shown in symbolic manner, e.g. as a colorreflecting a slew value or as an arrow reflecting the vector of theblood flow. The magnitude of the vector is the maximum slew rate of theblood flow pressure and the direction of the vector showing thedirection of the blood flow. Such imaging can be combined and alignedwith a visualization of the spatial progression and location of therespective vascular section. This can be realized by a method comprisingthe generation of imaging data reflecting the maximum slew rates asdescribed above, by an apparatus comprising a data processing unitadapted for generating these images or this imaging data, as well as bya computer program product, which realizes this method or this apparatuswhen running on a processing unit. The apparatus can also comprise avisual display for imaging the data. The imaging and the imaging data(which is preferably adapted to be displayed on a monitor) represents avector field, the position (or origin) of the vectors corresponding tothe respective locations of the respective particular vascular sections,the direction of the vectors corresponding to the respective blood flowdirections of the respective particular vascular sections and themagnitudes of the vectors representing the slew rate. The magnitude canbe represented by the lengths and/or colors of the arrows depicting thevectors. Preferably, the vector field is displayed together with anillustration of the corresponding vascular sections, the illustrationbeing aligned with the vector field.

The output element outputs the maximum slew rate or a value representingthe maximum slew rate as an indicator for the arteriovascular condition.The value representing the maximum slew rate can be normalized by anormalizer of the indicator device in order to provide thearteriovascular condition. As normalization factor, a predefined orinput value can be used. Further, the normalization factor can beprovided by a mean value of the flow condition. Such a mean value can beprovided by an external unit or, preferably by the averaging unit whichaverages successive values representing the maximum slew rate, themaximum slew rates each being related to one peak. Further, as anormalization factor, the ratio of this mean value to a cross sectionaldiameter can be used. The cross sectional diameter can be provided as aninput signal or can be predefined. In addition, the cross sectionaldiameter can also be represented by the cross sectional area or othervalues representing dimension of the pertaining vessel. Alternatively,the mean value can be provided by a normalization base input of theindicator device.

According to an aspect of the invention focused on the time ofoccurrence of the maximum slew rate, a control device is provided. Sucha control device provides an activation signal according to the time ofoccurrence of the maximum slew rate. As described above, a slew ratemonitor as well as a maximum detector is provided, similar to the slewrate monitor and the maximum detector as described above. However, ifthe maximum detector receiving the actual slew rate identifies theoccurrence of a maximum for the slew rate, a signal to an activationoutput of the control device is provided. Thus, the maximum detector isconnected to the activation output, wherein the activation output issuited to output an activation signal upon receipt of a signal (of a newmaximum slew rate) provided by the maximum detector. Again, for anactive training of the arteriovascular condition, it is important toidentify the point of time at which the maximum slew rate occurs forcoordinating actions on the pertaining arteriovascular section inresponse to the activation signal. Further, the activation output can besynchronized with the points of time at which the maximum slew rateoccurs. The activation signal can be provided with a predefined phasedifference to the point of time at which the maximum slew rate occurs.In addition, the activation signal can be delayed to the point of timeat which the maximum slew rate occurs by predefined delay. The delay aswell as the phase difference can reflect the behavior of an actuatordevice controlled by the activation signal and can reflect the desiredactivation time with regard to the point of time at which the maximumslew rate occurs.

According to a further aspect of the invention, an actuator device isprovided, which is adapted for being controlled by the activation signalprovided by the control device (or by the indicator device, if theindicator device provides a signal of the point of time at which theslew rate occurs). The actuator device comprises both, an actuatorsurface as well an actuator element adapted to move the actuatablesurface. The actuator element is connected to an input of the actuatordevice at which an activation signal is received. Thus, an activationsignal applied to the input of the activator device initiates a movementof the actuatable surface by the actuator element according to the pointof time at which the maximum slew rate occurs. Thus, the actuator deviceis adapted to be actuated at a frequency corresponding to the bloodpressure signal of the subject. Since the (positive) maximum of the slewrate is given as indicator for activation, the arteriovascular conditionis determined by the slope of the systole. The same applies to the pointof time at which the maximum slew rate occurs. The actuator device istriggered by the occurrence of the maximum slew rate. Between triggerand occurrence, a constant phase difference or time difference can beprovided. The actuator device is synchronized to the occurrence of themaximum slew rate. The actuator applies an external pressure onto thevascular section in synchronization with the occurrence of thedetermined maximum. The actuation can be delayed or can be advanced intime. Actuations in advance require synchronization by previousmeasurements of the time of occurrence of the maximum slew rate. Thedelay (or the advance in time) can be predetermined or can be a functionof the heart beat frequency reflected by the temporal progression, e.g.a monotonically increasing function.

The control device, together with the actuator device performs a methodfor stimulation of arteriogenesis. Such method for stimulation ofarteriogenesis comprises to exert pressure onto the arteriovascularsection or tissue to be treated in response to the determination that amaximum slew rate occurs. The pressure can be applied at the point oftime at which the maximum slew rate occurs. As an alternative, theapplication of pressure is initialized a predefined time before theoccurrence of the maximum slew rate or a predefined time afteroccurrence of the maximum slew rate. This corresponds to a phase shiftbetween the detection of the maximum slew rate and the application ofpressure. By the application of pressure, the slew rate resulting fromthe systole is supported by an additional external application ofpressure. By the synchronization of occurrence of the maximum slew ratewith the activation of the actuator device the additional pulses,resulting from the actuator are aligned to the slew rate measured in thearteriovascular section. This increases the maximum slew rate andtherewith increases the training effect, i.e. the enhancement of thearteriogenic capacity. Thus, arteriogenesis is trained by determiningthe maximum of the slew rate resulting from the systole, activating theactuator device according to the occurrence and therewith increasing theslew rate within the arteriovascular section by external stimulation(=application of pressure). In an alternative embodiment, the slew rateof this systole is determined by sensing electrodes according to anelectrocardiogram device. In the same way, the control deviceidentifying a point of time at which the maximum slew rate occurs cancomprise an input receiving an input signal representing aninstantaneous arterial blood flow condition not at the pertainingarterial vascular section but within the heart. Since the point of timeat which the maximum slew rate occurs within the arteriovascular sectioncorrelates with the heartbeat, measuring the heartbeat is an alternativeif only the time at which the maximum slew rate occurs is necessary.Thus, for synchronization of the actuator device with a point of time atwhich the maximum slew rate occurs, ECG-electrodes can be used (forexample within the inventive control device) for determining the pointof time at which the maximum slew rate occurs within the arteriovascularsection, i.e. the point of time to which the actuation of the actuatordevice correlates.

Consequently, the present invention also relates to a method forstimulation of arteriogenesis, comprising: determining a temporalprogression of an instantaneous blood flow condition of the arterialblood flow through an arteriovascular section of a subject; deriving aslew rate of the temporal progression during an increase of the temporalprogression; and determining the maximum of the slew rate, wherein anexternal pressure is applied repeatedly to the arteriovascular sectionin synchronization with the occurrence of the determined maximum.

Preferably, said external pressure is repeatedly applied to thearteriovascular section by an external counterpulsation (ECP) devicewhich is controlled by an arteriovascular condition indicator or controldevice. ECP is known in the art (see U.S. Pat. No. 6,191,111). Apossible arteriovascular condition indicator or control device which canbe used according to this aspect of the invention is the arteriovascularcondition indicator device or the arteriovascular condition controldevice as described above.

In a preferred embodiment, the arteriovascular condition indicator orcontrol device is for determining a relative peak shear increase (RPSI)and/or a pulse shear index (PSI) as described above.

The methods and devices of the present invention can be applied both onhuman and animal patients.

The methods and devices of the present invention can also be applied inorder to determine whether the introduction of catheters e.g. in thelegs or in the heart region has been successful, because such operationsresult in changes of arteriovascular conditions which can be determinedas disclosed herein.

Studies have been carried out proving the increased arteriogenesis bythe inventive training method. In one study, “Direct evidence fortherapeutic induction of arteriogenesis in patients with stable anginapectoris via external Counterpulsation” by Eva-Elina Buschmann et. al.,Franz-Volhard-Klinik, Max Delbrück Center, Helios Klinikum Buch, Berlin,Germany, 24 Patients (age 51-71) with stable coronary artery diseasehave been enrolled. One group underwent 35 1-hour sessions of trainingwith the actuator device within 7 weeks. The other group has not beentrained and forms the control group. The collateral flow index (CFI)reflecting the collateral circulation has been measured continuously.The CFI of the trained group has increased significantly from 0.09±0.017to 0.14±0.018 (p=0.018), whereas the control group did not showsignificant improvements (p=0.003). Further, a significant reduction ofthe CCS classification occurred for the trained group, in contrast tothe non-trained group (stable classification). Additionally, the studyshowed a significant increase of the D/S ratio (peak diastolicamplitude/peak systolic amplitude) from 0.9 (±0.06) to 1.1 (±0.05).

During further clinical studies, it has been demonstrated that the PSIvalue also correlates with the diameter of the respective arterial bloodvessel (see FIG. 10). This could also be confirmed in chicken embryos(FIG. 11). In a patient with shunt operation, however, the RPSI remainedconstant in view of an increasing diameter due to the increased bloodflow in this patient (FIG. 12).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a system suitable for carrying out theinvention;

FIG. 2 a shows a typical temporal progression of blood flow velocity and

FIG. 2 b shows a corresponding diagram of the first derivative of FIG. 2a with regard to time.

The differentiator 20 provides a derivation of its input with respect totime at an output of the differentiator 20. The system of FIG. 1comprises a measuring device 10, for example a sensor together with adata acquisition unit, which is connected to a differentiator 20. Thus,the signal output by the measuring device 10 is differentiated withrespect to time t. Therefore, the measuring device provides aninstantaneous blood flow condition, for example a blood flow velocity.The differentiator 20 forms the slew rate of the temporal progression ofthe blood flow velocity provided by measuring device 10 as a signal. Theslew rate 20 is applied to a maximum detector 30 which identifies andprovides the (positive) maximum of the slew rate provided by thedifferentiator 20. This maximum of the slew rate, preferably togetherwith the time of occurrence of the maximum is provided to a memory 40which is used as a buffer for the maximum values provided by the maximumdetector 30. Further, the system of FIG. 1 comprises an output element50 which represents the maximum slew rate as stored by the buffer 40 atan electronic display. Preferably, the output element 50 comprises anumeric display which displays the current maximum slew rate. In anotherembodiment, the memory is combined with an averaging unit, thecombination 40 receiving successive maximum values and forming theaverage of the received values. The resulting average is stored in thememory and retrieved by the output element 40 (or is supplied to theoutput element 50). According to a preferred embodiment, the combination40 of the average and the memory forms the average of a fixed number ofpreceding average values. For example, the averager provides the meanvalue for the last twenty of fifty values supplied by the maximumdetector 30. Further, the averager or the combination 40 of averager andmemory can be adapted to form a sliding window average of the mostrecent N values, N being a positive integer. According to an alternativeembodiment, the deviation unit 20 is a slew rate monitor which providesthe slew rate (corresponding to the derivative) as a ratio between anincrease and a respective distance of time spent by the increase. Thus,the deviation unit 20 forms a deviation of time discrete values.

The system of FIG. 2 can be provided by circuitry and/or by programmablehardware. In particular, some or all functions of the respective blockscan be provided as software modules. In this case, the connectionbetween the individual blocks is provided by a module header or afunction header comprising input and/or output variables. Preferably,block 10 comprises an analog/digital converter and/or following blocks.In particular blocks 20, 30 and 40 are provided by a microprocessorcontrolled by software.

FIG. 2 a shows the blood velocity in the course of time. In FIG. 2 a,three heartbeats are shown in form of pulses each having a rising edgeresulting from the systole. The progression of the blood flow condition(in this case the blood flow velocity) is represented by time discretemeasuring points each measuring point shown as a dot. In addition, aline has been added connecting all dots in succession. As can be seenfrom FIG. 2 a, the maximum slew rate of the first pulse is at t1, themaximum slew rate of the second pulse is at t2 and the maximum slew rateof the third pulse is at t3. These maximum increases represent thearteriovascular condition. In FIG. 2 a, the blood velocity values aregiven as time discrete values reflecting individual measuring points.The time duration between two successive measuring points is equal forall successive points according to a sampling data acquisition procedurewhich periodically provides an measuring point according to the samplingrate of the data acquisition device, for example ananalog/digital-converter, in particular a sample and hold unit thereof.

In FIG. 2 b, the derivation with respect to time of the progressionshown in FIG. 2 a is given. The differentiation with respect to time isreflected by the symbol wherein v is a physical quantity (the velocity)and v′ is equal to dv/dt. In a general aspect of the invention, dv/dtcan be a differential quotient with dt=t−t₀→0 or with dt=t−t₀=Δt>0. Thepoints of time t1, t2 and t3 are identical to the points t1, t2 and t3of FIG. 2 a. At t1, the maximum slew rate is given as v′1, at t2, themaximum slew rate is v′2 and the maximum slew rate of the third pulse isgiven at v′3. According to the time discrete nature of the values shownin FIG. 2 a, the derivation values in FIG. 2 b are also time discreteand are represented as squares, together with a line connecting thesesquares. The derivation values shown in FIG. 2 b are given as ratiobetween the increase shown in FIG. 2 a (difference between two adjacentvalues) and the time distance between two successive points, that is thetime interval corresponding to the sampling rate shown in FIG. 2 a. Thederivation at a point of time is equivalent to the difference betweenthe physical quantity right before and right after the point of timedivided by the time between both values (or multiplied by the samplingrate). Thus, a point reflecting the derivation always lies between thetwo data points or measuring points adjacent thereto.

FIG. 3. In vivo measurements of red blood cell velocities in yolk sacarteries and veins.

(A) Overview of yolk sac vascular network in vivo. (B) Schematicrepresentation of the vascular plexus. Arteries indicated in red, veinsin blue. A1-A4 indicate the measurement sites in arteries, V1-V4 inveins. (C,D) red blood cell velocity profiles, and (E,F) thecorresponding first order derivative of velocity−acceleration rate(dv/dt), in arteries and veins. Dotted line (C) indicates first orderderivative for the time point of fastest acceleration (peak velocityincrease, PVI). H, heart; VA, vitelline artery; VV, vitelline vein; SV,sinus vein. Scale bar, 3 mm in A.

FIG. 4. Identification of flow signals that discriminate arteries fromveins.

(A) time averaged mean red blood cell velocity increases with diameterin yolk sac arteries and veins. (B) mean shear rates are relativelyconstant for both arteries and veins.

Regression line (solid lines) and 95% confidence interval (dotted lines)are indicated; regression coefficients are not significant. (C) redblood cell acceleration rate in arteries and veins. (D) accelerationrate as a function of mean velocity separates arterial from venousvessels. (E) acceleration rate normalized to mean velocity (relativepulse slope index—RPSI) as a function of diameter, yields separation ofarterial (top) and venous (bottom) domain. Optimal separation ofarterial from venous vessels occurs at a cutoff of RPSI=7.9 s⁻¹. (F)RPSI in arteries and veins. ***p<0.001; MannWhitney U-test.

FIG. 5. Role of oxygen in yolk sac remodeling.

(A) in vivo measurements of oxygen saturation shows significantly lowerlevels in arteries compared to veins. (B) exposing developing embryos toan ambient oxygen level of 10% O₂ (hypoxia) impairs yolk sac arterialnetwork growth (red arrow) compared to exposure to 21% O₂ (normoxia).LVA, left vitelline artery; RVA, right vitelline artery.

FIG. 6. Flow redistribution after right vitelline artery (RVA) ligation.

(A,B) Schematic representation of flow distribution in the yolk sacposterior pole, before (A) and after (B) ligation of the right vitellineartery (RVA). Note the reversal of flow direction in the RVA distalarterioles. In proximal and distal arterioles of the left vitellineartery (LVA), mean velocities (C,E) and mean shear rates (D,F) increasedafter RVA ligation, indicating that flow is shunted from the RVA to theLVA. *p<0.05, **p<0.01 pre-versus post ligation, Student's t-test. VV,vitelline vein; p, proximal arteriole; d, distal arteriole. Black arrowsindicate normal flow direction, purple arrows the changed directions.

FIG. 7. Flow redistribution results in formation of a collateralarterial network.

Acutely after ligation FITC-dextran-angiography (A-D) indicates aperfusion deficit on ligated side (A). Asterisk indicates site of rightvitelline artery ligation. Within 10 minutes post-ligation (B,C), bloodstarts to flow from the posterior venous domain towards the ligated sidevia pre-existing RVA arteriolar channels (D). Time-lapse imaging of thisarea (E-H) shows the rapid formation of an arterial network through thepreviously venous domain. At 24 h post ligation (I-K) a significantnumber of perfused large caliber arteries can be detected in ligationembryos, not in controls (K). Scale bars: 4.9 mm in A, 1.8 mm in B, 3.1mm in C; 3.6 mm E-H; 2.4 mm in I.

FIG. 8. Time lapse imaging of collateral arterial network formationafter ligation (A-H) Still images from time-lapse movie show how thepre-existing RVA arteriolar segments at the ligated side (left part ofimage) enlarge, while the vitelline vein is progressively pruned. Thepruning of the vein allows the LVA to expand through the former venousterritory. With time, the thoroughfare channels obtaining the highestflow enlarge further and establish the collateral arterial network. Notethe structural diameter increase of terminal arterioles of the LVA (redasterisk). Arrows indicate flow direction in vein (blue) andpre-existing arterioles (red). Flow direction in RVA is reversed in postcompared to preligation. Yellow arrowhead indicates the pruned vitellinevein. LVA, left vitelline artery; RVA, right vitelline artery.

FIG. 9. Adaptive lumen changes in the stem of the left and rightvitelline artery after ligation. In vivo imaging of arterial diameteradaptation of right and left vitelline artery in control (A,C) andligation (B,D) embryos shows progressive pruning of the right vitellineartery segment proximal to the ligation (F). The contralateral leftvitelline artery in the ligation embryo now receives more flow anddisplays outward remodeling (E). LA, left vitelline artery; RA, rightvitelline artery. *, p<Ø.Ø5; ***p<Ø.ØØ1. Scale bars: Ø.62 mm in A,B;1.25 mm in C,D.

FIG. 10. Correlation between arterial diameter and RPSI in healthypatients.

The RPSI declines with increasing arterial diameter.

FIG. 11. Correlation between diameter and RPSI in young and old chickenembryos.

The RPSI declines with increasing arterial diameter.

FIG. 12. Correlation between arterial diameter and RPSI in a shuntoperated patient

The RPSI remains constant in view of the increasing blood flow.

FIG. 13. Receiver Operator Characteristics Analysis.

Results of a Receiver Operator Characteristics (ROC) analysis, of therelative pulse slope index (RPSI), for the discrimination of arteriesfrom veins. The top panel gives the ROC curve with an ROC area under thecurve (AUC) of 0.98 (SE=0.01 1, p<0.0001). An AUC of 1 indicates aperfect separation while a random selection (corresponding to thediagonal dashed line shown in the graph) exhibits an AUC value of 0.5.The middle panel shows the dependence of accuracy of discrimination ofarteries from veins from the chosen critical RPSI value. The accuracy ofdiscrimination is calculated by the Youden index (J=max[SEi+SPi−1] whereSEi and SPi are the sensitivity and specificity for a given RPSIvalues). The maximum accuracy value of J=0.8928 is achieved for an RPSIof 7.909 (circle) is marked by a circle in both panels. Thediscrimination of arteries and veins by this RPSI value (gray horizontalline, 7.9) is shown on the dot diagram in the lower panel. RPSI is givenin s⁻¹.

EXAMPLE 1. Introduction

Arterial and venous vascular networks show a distinct genetic signature,function and branching architecture (De Smet et al., 2009; Swift andWeinstein, 2009). Specification of arterial-venous vessel identity andformation of branched vascular networks occur during early embryogenesisand are modulated by hemodynamic factors (Jones et al., 2004; le Nobleet al., 2004; Lucitti et al., 2007), but the precise mechanisms areunclear. Circulation of blood creates mechanical forces in vessels(Garcia-Cardena et al., 2001; Jones et al., 2006), and affectsoxygenation of developing organs. Here we investigated which mechanicalforces, or secondary factors including oxygenation of the blood (Fraislet al., 2009), might be relevant for regulating vessel identity indeveloping embryonic vascular networks in vivo. We furthermore assessedthe morphological and genetic changes occurring in the embryonic yolksac vasculature in response to manipulations of hemodynamic conditions,and show that genes strongly regulated herein, might also exert afunctional role in collateral arterial network growth (Buschmann andSchaper, 1999; Schaper, 2009) during pathological conditions. In theembryo, vascular branching morphogenesis and vessel identity can beregulated by two distinct mechanisms: genetic hardwiring of vesselpositioning and identity, and hemodynamics controlled vascularpatterning and maintenance of vessel identity (Jones et al., 2006).Hardwiring of vessel positioning at the capillary level involvesendothelial tip cells, and occurs independent of flow (Gerhardt et al.,2003; Hellstrom et al., 2007). Arterial specification requiresactivation of sonic hedgehog (shh)/VEGF/neuropilin-1/Notch pathways(Lawson et al., 2001; Lawson et al., 2002; Swift and Weinstein, 2009;Zhong et al., 2001). In the chick embryo manipulation of hemodynamicparameters changed the global patterning of arteries and veins in theyolk sac vasculature (le Noble et al., 2004).

In this example, we considered factors related to flow, pressure andoxygenation and performed a comprehensive in vivo analysis of thesefactors, substantiated by in vitro experiments. We found a uniqueparameter related to the pulsatility of blood flow, the relative pulseslope index (RPSI), which distinguishes arterial from venous domains.

2. Materials and Methods Chick Embryos:

Fertilized chick (Gallus gallus, white leghorn) embryos were purchasedfrom commercial sources and incubated at 38° C. in a humidifiedatmosphere. Embryo stages were determined according to the number ofsomites formed. Handling of the embryos and ligation of the rightvitelline artery was performed as described previously (le Noble et al.,2004). FITC-Dextran (Sigma, Mw 200 kDa, 8 mg/ml in PBS) to visualizeplasma flow was injected intravascular using a micropipette.

Flow Driven Models

The three vessels occlusion brain arteriogenesis (3-VO) model in maleSprague Dawley (SD) rats was performed as described (Busch et al., 2003;Buschmann et al., 2003). In short: both vertebral arteries were occludedvia electrocoagulation. During the further occlusion of the left commoncarotid artery, cerebral blood flow was measured by laser Dopplerflowmetry to ensure cerebral hypoperfusion. Three weeks after 3-VO, ratswere anaesthetized and cerebral blood flow was measured via LDF afterinducing maximal vasodilation with acetazolamide. To visualize thearteries of the circle of Willis colorized latex (Chicago LatexProducts, no. 563) was perfused via a catheter into the maximallydilated (with Papaverin) cerebral arterial circulation. Vessel diameterswere measured under the microscope.

The gap junction uncoupler carbenoxolone (Sigma) diluted in 0.9% NaClwas administered i.p. at a dosage of 1.184 mg/day (C57/Bl6 mice) or 1.48mg/day (SD rats) for a period of 7 or 21 days respectively; controlsreceived 0.9% NaCl. All animal experiments were approved by the localethics committee.

In Vivo Microscopy: Time Lapse Imaging, Measurement of Red Blood CellVelocity and Oxygen Saturation

In vivo time-lapse imaging and intra vital video-microscopy wereperformed as described (le Noble et al., 2004; Lindert et al., 2002). Inshort: yolk sac blood vessels were imaged using a 25× objective (NA 0.6)and an asynchronous strobe light illumination (Lindert et al., 2002).This illumination generates image pairs with a time delay (delta t) ofdown to 0.5 ms. Using a spatial correlation approach, the spatialdisplacement (delta 1) of the red cell column during this delay isdetermined off-line. The flow velocity is then calculated as V=delta1/delta t with a temporal resolution of 25 Hz for velocities up to 40mm/s. For the determination of oxygen saturation, a multispectralapproach was used (Styp-Rekowska et al., 2007).

Statistical Analysis

Data are expressed as the means±SEM. P values were calculated usingStudents t test or Mann Whitney U test (for non-normal distributions). Ap value <0.05 was considered statistically significant.

3. Results 3.1 In Vivo Imaging of Blood Flow Parameters and Oxygen inChick Embryo Arteries and Veins

3.1.1 Adaptation to Shear Stress, Identification of Relative Pulse SlopeIndex (RPSI) as a Parameter to Discriminate Arteries from Veins

Red blood cell velocities (vRBC) were measured in arteries and veins invivo (FIGS. 3 a, 3 b). We noted the striking pulsatility in the flowvelocity profile in arteries (FIG. 3 c) compared to the more constantflow velocity in veins (FIG. 3 d). From the velocity profiles wecalculated the red blood cell acceleration rate (1^(st) orderderivative, dv/dt, in mm/s², see red line in FIG. 3 c) during each heartcycle. Note the steep acceleration rates during the initial systole inarteries (FIG. 3 e), not in veins (FIG. 3 f).

We next examined which flow related parameter discriminates arteriesfrom veins (FIG. 4). In both arteries and veins, mean red blood cellvelocity increased with increasing lumen diameter (FIG. 4 a). Thebiological relevant parameter for vessel lumen adaptation to blood flowis the shear acting on the endothelial surface. Shear is proportional tothe flow velocity with the proportionality factor depending inversely onthe vessel diameter (mean velocity/vessel diameter). We estimated thetime averaged (mean) shear rate as mean centerline flow velocity dividedby vessel diameter (FIG. 4 b). For arteries, (diameter range: 24-180 m;n=65 vessels; 10 animals) the typical mean shear rates ranged from 2(s⁻¹) to 9 (s¹). Venous mean shear rates were slightly higher, averagingbetween 6-30 (s⁻¹) (n=60 vessels, 10 animals). These levels of shearrate are generally assumed to correlate to non-turbulent, laminar flowin microvessels. Regression line analysis (lines in FIG. 4 b) revealedthat the slope of the regression line was not significant for botharteries (r²=0.03 5) and veins (r²<0.006) (FIG. 4 b). This indicatesthat both yolk sac arterioles and venules adapt their lumen diameter tomaintain constant shear rates. Since shear rates of arteries and veinsshowed overlap, shear rate itself did not discriminate sufficientlybetween arteries and veins.

We next considered the acceleration rate (FIG. 4 c). The peak velocityincrease (PVI, dv/dt max) represents the maximal acceleration rate ofthe red blood cells, occurring in the early systole. The PVI increasedwith increasing diameter in both arteries and veins (FIG. 4 c). In thesmaller diameter range (<100 m) there was considerable overlap betweenacceleration rates obtained from the arterial or venous domain, and PVItoo, did not discriminate sufficiently arteries from veins.

However, PVI in a vessel proved to vary systematically as a function ofthe mean velocity in that vessel (FIG. 4 d). It was possible to achievean almost complete separation of the arterial and venous domain by aline with slope 1 and a relation of PVI to mean velocity of 7.9 (seeFIG. 13; Receiver Operator Analysis).

This observation suggests that the quotient PVI/mean velocity is suitedto discriminate arteries from veins (FIG. 4 e). The resulting parameteris referred to as relative pulse slope index (RPSI=PVI/mean vRBC ins⁻¹). RPSI was significantly higher in arteries compared to veins withalmost no overlap between the two domains, at a cut-off value of 7.9 s⁻¹(FIG. 4 f, FIG. 13). Arteries have RPSI values exceeding 7.9 with 99%confidence.

3.1.2 Cyclic Stretch is Limited to the Aorta

In the adult, the distensible nature of the arteries averages out thepressure pulsations, allowing a continuous flow in the distal parts ofarterial tree. We measured vessel distension (cyclic stretch) during theheart cycle in the dorsal aorta, and the yolk sac vitelline arteries andvenules including the veins of the inflow tract. A small but significantamount of cyclic stretch (2.93%±0.57, n=23) was noted in aorta. In thevitelline arteries and venules, distension was not detectable in allanimals investigated (n=15 animals, 5-6 arteries or veins per animal).The lack of distensibility supports pulsatile flow up to the distalparts of the yolk sac arterioles.

3.1.3 Oxygen Measurements in Arteries and Veins, Hypoxia Challenge InVivo

The primary function of the yolk sac circulation is to take up nutrientsfrom the yolk and allow gas (O₂, CO₂) exchange, equivalent to placentafunction in mammals. In line with placenta function, we observedsignificantly lower (p<0.001) oxygen saturation levels in arteries(63.2%±1.9%) when compared to veins (78.4±3%, n=6; FIG. 5 a). We nextincubated chicken embryos in hypoxic conditions and examinedarterial-venous network development (FIG. 5 b). Exposure to an ambientoxygen level lower than 10% O₂ was embryonic lethal. Exposure to 10% O₂induced growth retardation, cardiac malformations (in 7 out of 8embryos), and bradycardia, when compared to age-matched normoxic (21%O₂) controls. In hypoxic embryos, the complexity of the arterialnetwork-size of the vessels, and distal/lateral expansion of thenetwork—was significantly reduced compared to normoxic controls (FIG. 5b). The adverse effects of hypoxia on embryonic cardiac function (Tintuet al., 2009), resulting in perfusion deficits may contribute to theimpaired vascular development.

3.2 Flow Driven Macroscopic and Microscopic Changes in the ArterialNetwork

We next investigated how embryonic vessels adapt structure and branchingpattern in response to changes in flow distribution using the chickembryo ligation model.

3.2.1 Increased Flow Velocities and Shear Rates in the Left VitellineArterial (LVA) Network after Ligation of the Right Vitelline Artery(RVA)

We first quantified the flow changes occurring in the proximal anddistal parts of the left vitelline artery (LVA) after ligation of theright vitelline artery (RVA; FIGS. 6 a,b; p indicates proximal,d—distal, measurements were made in the same vessel pre- and postligation). In the LVA proximal arteries, mean velocities (FIG. 6 c)increased significantly from 1.76±0.1 mm/s pre-ligation to 3.3±0.4 mm/spost ligation (n=12, p<0.05). We observed no acute diameter change inthese arteries (% change pre-versus post-ligation was 1.02%, ns). Meanshear rates increased from 4.7±0.7 s⁻¹ pre-ligation to 6.9±1.1 s⁻¹post-ligation (n=12, p<0.05; FIG. 6 d). In the LVA distal arteriolesmean velocities (FIG. 6 e) significantly increased from 0.23±0.03 mm/sto 0.53±0.1 mm/s (n=12, p<0.05). Again, no acute diameter changes wereobserved. Also local application of acetylcholine (vasodilator) ornorepinephrine (vasoconstrictor) did not affect diameter, indicatingthat these vessels have no acute regulation of vasomotor tone. The meanshear rates significantly increased from 3.3±0.6 s⁻¹ to 5.7±0.6 s⁻¹(FIG. 6 f). These data show that ligation of the RVA increased perfusionof the LVA arterial network up to the most distal branches.

3.2.2 Arterial Patterning Follows the Redistribution of Blood Flow

Redistribution of blood flow upon RVA ligation was evaluated usingFITC-dextran angiography (FIGS. 7 a-d). Acutely after ligation, theligated right side showed a clear perfusion deficit; the LVA was wellperfused (FIG. 7 a). Within 10 minutes after ligation, some blood flowwas recruited to the right side via retrograde perfusion of RVAarterioles in the posterior pole (FIGS. 7 b,c). This blood flow wasderived from the anterior venous plexus. Thus after ligation, bloodflows from the LVA network, through the posterior vitelline vein domain,into the pre-existing arterioles of the RVA arterial network back to theheart (FIG. 7 d). In these RVA arterioles the blood flow direction isreversed compared to pre-ligation, and pulsatility was reduced. RPSIvalues in ligated arterioles (n=5) dropped significantly from 11±0.5(arterial domain) to 1±0.25 s⁻¹, thus showing venous flowcharacteristics. The reversal of flow direction in posterior pole wasfurthermore confirmed with intravital microscopy (supplemental movie 1versus supplemental movie 2: note both changes in direction, andvelocity). In the RVA pulsatility is lost after ligation because theblood flow used for perfusing it, comes from the LVA, and has to travelthrough the capillaries in the posterior pole that due their smalldiameter and high resistance dampen the pulsatile flow component.

Within 15 hours, the changes in flow distribution, associated with aglobal change in arterial patterning (FIGS. 7 e-h). The LVA arterialnetwork expanded towards the ligated right side with branches growingthrough the territory normally occupied by the veins, and projectingtowards the right side of the embryo (FIGS. 7 e-h). Concomitantly, theposterior vitelline vein regressed. At 24 hours post ligation, anelaborate collateral arterial network restoring flow to the occludedside was established in all embryos investigated (FIGS. 7 i,j).Collateral arteries were defined by a clear anatomical connection to theLVA, carrying an arterial flow profile, and crossing the embryo/yolk sacmidline (FIG. 7 i,j). In ligated embryos the number of collateralarteries ranged from 3 to 20 (median=7, n=12 animals), in controlembryos such collateral arteries were never observed in all animals(n=14) investigated (FIG. 7 k).

We next imaged the microvascular changes in vivo using time-lapseintravital microscopy (supplemental movie 3, still images in FIG. 8).Acutely after ligation, blood flow coming from the terminal part of theLVA (FIG. 8 a, red arrows) was distributed towards the vitelline veins(FIG. 8 a, blue arrows) and the arterial network on the ligated sideusing preexisting vessel segments (FIG. 8 a, small red arrows,supplemental movie 3). The new flow direction (in supplemental movie 3,from the right side of the panel towards the left side) is perpendicularto the flow direction normally occurring in the venous territory. Withtime, the amount of flow attracted towards the ligated side increased,at the expense of flow entering the vein. The reduced flow toward thevein, associated with diameter reduction, and subsequent pruning of thevein (FIGS. 8 a,c,e,g; yellow arrowhead). In contrast, the increasedflow through the LVA distal arterioles and pre-existing segments on theligated side induced a diameter increase in these vessels (FIG. 8, redasterisk).

3.2.3 Adaptive Arterial Diameter Remodeling after Right Vitelline Artery(RVA) Occlusion

Ligation of the RVA resulted in priming of the arterial segment proximalto the ligation and outward remodeling of the comparable arterialsegment on the contralateral side (FIG. 9). At 5 hours post-ligationlumen diameters of the stem of the RVA were significantly smaller(ligation 140±20 tm versus 258±22 tm in time matched controls, p<0.001,n=6, FIGS. 9 b,f) and after 24 hours, the right arterial segment wasanatomically not detectable in all animals investigated (FIGS. 9 d,f).In contrast, the arterial segment of the LVA showed increased diametersgrowth (FIGS. 9 a-d) which was already detectable 5 hours post ligation(ligation 289±30 tm versus time matched control 252±22 tm; n=6 embryos,p<0.05, FIGS. 9 a,e); and more pronounced after 24 hours (ligation381±22 tm versus control 299±12 tm; n=6 embryos, p<0.05, FIGS. 9 c,e).Thus, ligation of the RVA, results in shunting of flow to the LVAnetwork causing outward remodeling in this area.

4. Discussion of the Example

Blood flow is needed to deliver oxygen to growing tissues in the embryobut also generates biomechanical forces including shear and wall stress(Jones et al., 2006). We performed an extensive in vivo analysis ofartery-venous specific characteristics related to flow, pressure, andoxygen availability, and found that it is most likely that shearmediated signals contribute to regulation of arterial identity andremodeling.

We show that yolk sac arterioles and venules adapt their lumen size tothe amount of flow carried as evidenced by maintenance of relativelyconstant shear rates, and rapid adjustment of lumen diameter in responseto changes in flow. In artery occlusion experiments arteries exposed toincreased flow, increase their diameter whereas pressurized arterialsegments that don't carry flow regress. Ligation of the right vitellineartery induces the formation of a collateral arterial network branchingfrom the contralateral left vitelline artery, transporting blood flow tothe hypoperfused occluded side. This collateralization process involvesflow driven “upgrading” of capillary segments into arterioles guided bythe flow gradient. Within 24 hours it results in complete restoration ofblood flow to the ligated area. Since acute vasomotor responses were notobserved, the diameter changes occurring post ligation, are ofstructural nature. Anatomical properties of yolk sac arteriolesincluding an incomplete vessel wall may facilitate this rapid structuraladaptation.

We noted striking differences in flow pulsatility between arteries andveins in vivo. Theoretically, the steepness of the shear increase duringthe early systole would be the strongest pulsatile signal available toendothelial cells. The best separation between arterial and venousvessels was indeed obtained by estimating this signal, i.e. the maximalpositive change in shear rate relative to the time averaged shear ratein the same vessel called relative pulse slope index, RPSI [s⁻¹].Arteries have RPSI values exceeding 7.9 with 99% confidence, while therespective probability is only 5% for veins. Of course theseobservations don't prove causality and for RPSI to be physiologicallyrelevant, endothelial cells have to sense fluctuations in shear (Dai etal., 2004; Garcia-Cardena et al., 2001). We show in vitro that arterialendothelial cells exposed to pulsatile shear maintain expression of thearterial marker ephrinB2, which was not observed with constant shear. Invivo, RPSI above 7.9 correlated well with arterial marker expression.Right vitelline artery (RVA) ligation, caused RPSI in perfused vesselson the ligated side to drop from the arterial (above 7.9) to venousdomain (below 7.9) This suggests that in this setting, pulsatileflow/shear, not pressure, regulates arterial identity genes.

Although our observations offer some explanation for arterial-venousfate control by flow, relevant questions remain. Previous studies showedthat during early embryogenesis both arterial and venous endothelialcells can change their genetic identity, in response to alterations inlocal cues (le Noble et al., 2004; Moyon et al., 2001). However, withtime this plasticity is lost, through a yet unknown mechanism (Moyon etal., 2001). Adult veins exposed to arterial flow regimes loose venousidentity genes, but don't acquire arterial markers (Kudo et al., 2007).Our failure to induce an arterial phenotype in venous cells in vitro mayalso reflect this loss of plasticity. The vessels we studied are ratherimmature in both function and structure. In adult vessels, with anintact mature vessel wall, remodeling may require more time oradditional digestive actions. It is therefore clear that ourobservations can't be generalized for all conditions in the adult, andcontribution of signals from perivascular nerves has to be considered(Larrivee et al., 2009). In the chick embryo yolk sac, arterialpressures are extremely low and range from 0.4 mmHg (stage 14) to about1.35-0.8 mmHg (stage 23) (Girard, 1973; Van Mierop, 1970; Van Mierop andBertuch, 1967). Venous pressures were lower than 0.2 mmHg. If absolutepressure values discriminate arteries from veins, endothelial cellsshould be capable of sensing a threshold pressure around 0.4 mmHg. Toour knowledge thus far no evidence showing such a value in the contextof arterial-venous differentiation in endothelial cells exists. Instead,some studies show that, in the adult arterial marker expression might bemodulated via pressure related cyclic stretch of the vessel wall (Korffet al., 2008). Our in vivo observations indicate that, if any, thecontribution of cyclic stretch in determining expression of identitygenes is limited to embryonic aorta and outflow tract.

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1. A method for determining an arteriovascular condition of a subjecthaving an arterial blood flow, comprising: determining a temporalprogression of an instantaneous blood flow condition of the arterialblood flow; deriving a slew rate of the temporal progression during anincrease of the temporal progression; and determining the maximum of theslew rate, wherein the arteriovascular condition is determined by avalue representing the maximum of the slew rate.
 2. The method of claim1, wherein determining the temporal progression of the instantaneousblood flow condition comprises a step of measuring at least one of thefollowing physical quantities: an instantaneous blood flow velocity, aninstantaneous fluid pressure or an instantaneous shearing force withinan arteriovascular section of the subject, the method further comprisinga step of determining the blood flow condition according to a predefinedfunction of at least one of the physical quantities.
 3. The method ofclaim 2, wherein the value representing the maximum of the slew ratecorresponds to (a) the maximum slew rate of the instantaneous blood flowvelocity, (b) the maximum slew rate of the instantaneous fluid pressure,or (c) the maximum slew rate of the instantaneous shearing force, or (d)one of these maximum slew rates normalized by a mean value of the bloodflow velocity, of the fluid pressure or of the shearing force or (e) oneof these slew rates normalized by the ratio of the mean value of theblood flow velocity, of the fluid pressure or of the shearing force tothe cross sectional diameter of the arteriovascular section or to afraction thereof.
 4. The method of one of the preceding claims, whereinderiving the slew rate of the temporal progression comprises:determining the slew rate by deriving the progression of the blood flowcondition with respect to time; or determining the slew rate by relatingan increase of the progression to the distance of time spanned by theincrease.
 5. The method of one of the preceding claims, wherein thetemporal progression is represented by a time-discrete blood flowcondition signal comprising at least two values of the same positiveslope or by a time-continuous blood flow condition signal comprising atleast one positive slope.
 6. The method of one of the preceding claims,comprising determining a measure for an arteriogenic capacity of thesubject, wherein the method comprises repeatedly determining a variationof the arteriovascular condition with a repetition in time intervalsgreater than 5 days, wherein the variation of the arteriovascularcondition is in monotone increasing relationship to a measure for anarteriogenic capacity of the subject reflecting the progress of anarteriovascular condition of the subject or reflecting a response onphysical training of the subject; wherein the measure for thearteriogenic capacity is determined according to the variation and therelationship.
 7. The method of one of the preceding claims, wherein thevascular condition is determined by a plurality of consecutiveindividual arteriovascular condition determinations and by averaging theresulting individual arteriovascular conditions, or by identifying themaximum of the resulting individual arteriovascular conditions.
 8. Themethod of one of the preceding claims, wherein determining a temporalprogression of a blood pressure quantity comprises measuring blood flowproperties within an arteriovascular section of the subject by laserDoppler velocimetry, sonography or magnetic resonance imaging of thearteriovascular section, or by pressure sensoring according to wristblood pressure monitoring, finger blood pressure monitoring,sphygmomanometry, plethysmometry, plethysmography, or intravascularblood pressure sensing, or impedance measurements of tissue or at thearteriovascular section or at a myocardial section.
 9. The method of anyone of the preceding claims, wherein a relative peak shear index (RPSI)and/or a pulse shear index (PSI) is determined.
 10. An arteriovascularcondition indicator device, comprising: an input (10) for receiving aninput signal representing an instantaneous arterial blood flow conditionof a subject; a slew rate monitor (20) connected to the input (10), theslew rate monitor (20) relating variations of the blood flow conditionto a distance of time spanned by the variation, the slew rate monitor(20) providing a slew rate representing the variations; a maximumdetector (30) connected to the slew rate monitor (20) for receiving theslew rate, the maximum detector (30) being suited to identify and toprovide a maximum slew rate to an output element (50) of the indicatordevice, the output element (50) being connected to the maximum detector(30) and being suitable to output a representation of the maximum slewrate as an indicator for the arteriovascular condition.
 11. Theindicator device of claim 10, wherein the slew rate monitor (20)comprises a differentiator adapted to provide a derivation of the inputsignal as the slew rate or wherein the slew rate monitor (20) comprisesa division unit adapted to divide an increase of the input signal by thedistance of time spanned by the increase and to provide the result asthe slew rate.
 12. The indicator device of claim 10 or 11, wherein themaximum detector (30) comprises a comparator suitable to comparesuccessive slew rates provided by the slew rate monitor with each otherand to provide the higher slew rate as the maximum slew rate to theindicator device (50) or to a memory of the indicator device adapted tostore a maximum slew rate for retrieval by the indicator device, thememory being connected to the maximum detector as well as to the outputelement.
 13. The indicator device of one of claims 10-12, furthercomprising an averaging unit (40), the averaging unit (40) beingconnected between the maximum detector (30) and the output element (50),the averaging unit (40) being suitable to average a plurality ofsuccessive maximum slew rates output by the maximum detector (30) and toprovide the average of the maximum slew rates to the output element(50).
 14. The indicator device of one of claims 10-13, furthercomprising a normalizer, the normalizer being suited to normalize theinput signal representing an instantaneous arterial blood flowcondition, the slew rate representing the variations, or the maximumslew rate by division by a normalization factor, wherein the input issuited for receiving values representing the blood flow condition as ablood flow velocity, a fluid pressure or a shearing force, thenormalization factor being provided by: (i) a mean value of the bloodflow condition as a result of an averaging unit; or by (ii) a ratio ofthe mean value of the blood flow condition to a cross sectional diameterof the arteriovascular section or to a fraction of the cross sectionaldiameter, the ratio being provided by a normalization base input of theindicator device.
 15. The indicator device of any of claims 10-14,wherein the device determines a relative peak shear index (RPSI) and/ora pulse shear index (PSI).
 16. A control device for providing anactivation signal comprising: an input for receiving an input signalrepresenting an instantaneous arterial blood flow condition of asubject; a slew rate monitor connected to the input, the slew ratemonitor relating variations of the blood flow condition to a distance oftime spanned by the variation, the slew rate monitor providing a slewrate representing the variations; a maximum detector connected to theslew rate monitor for receiving the slew rate, the maximum detectorbeing suited to identify a point of time at which the maximum slew rateoccurs, and to provide a signal to an activation output of the controldevice synchronized to the point of time of the maximum slew rate, theactivation output being suited to output the activation signal uponreceipt of the signal provided by maximum detector.
 17. An actuatordevice comprising a control device according to claim 14, the actuatordevice further comprising an actuatable surface as well as an actuatorelement mechanically connected thereto, the actuator element beingconnected to the activation output and being adapted to move theactuatable surface in response to the activation signal.
 18. A methodfor stimulation of arteriogenesis, comprising: determining a temporalprogression of an instantaneous blood flow condition of the arterialblood flow through an arteriovascular section of a subject; deriving aslew rate of the temporal progression during an increase of the temporalprogression; and determining the maximum of the slew rate, wherein anexternal pressure is applied repeatedly to the arteriovascular sectionin synchronization with the occurrence of the determined maximum.
 19. Amethod according to claim 17 wherein said external pressure isrepeatedly applied to the arteriovascular section by an externalcounterpulsation device which is controlled by an arteriovascularcondition indicator or control device.
 20. The method according to claim19, wherein the arteriovascular condition indicator or control device isfor determining a relative peak shear increase (RPSI) and/or a pulseshear index (PSI).
 21. The method according to claim 19, wherein thearteriovascular condition indicator or control device is as defined inany of claims 10 to 16.